A Plymouth Township-based biotech company founded by a scientist who co-discovered the best-selling pharameutical drug ever has filed documents to go public. Esperion Therapeutics, formed five years ago by Michigan scientist Roger Newton, who co-invented Lipitor in the 1980s, filed documents with the Securities and Exchange Commission on Tuesday. It marks the second potential public offering of the year for a Michigan company. On Tuesday, Compuware subsidiary Covisint filed its own documents to go public. Collectively, the news could signal that the gates to becoming a public company are opening for small tech companies in Michigan.